(1) Field of the Invention
The present invention relates to an apparatus for displaying television images applicable to automotive vehicles, particularly, relates to a technique for limiting an angle of a field of view from the driver's seat position of the vehicle so as to provide no recognition of the TV images from the driver's seat position.
(2) Description of the Background Art
A Japanese Patent Application First Publication No. Heisei 1-248,885 published on Oct. 4, 1989 exemplifies one previously proposed Television (hereinafter abbreviated to TV) image displaying apparatus applicable to automotive vehicles.
In the above-identified Japanese Patent Application First Publication, a liquid crystal filter is installed on a front portion of a TV monitor (or CRT (Cathode Ray Tube).
The TV monitor is installed on a center console of the vehicle compartment. The liquid crystal filter is controlled by means of a liquid filter control/drive circuit.
The liquid crystal filter is provided with a whole electrode on its whole surface. When the liquid crystal filter receives a voltage applied across the surface electrode, the whole surface becomes polarized (black as viewed from its front surface).
At this time, if a maximum direction of a contrast ratio of the liquid crystal filter is set toward eyes of the driver (one of viewers of the TV monitor), the images of the TV monitor cannot be viewed from the driver's position.
However, since the angle of (viewing) field of the liquid crystal filter is narrowed directionally, the TV images can still be viewed from the passenger seat side.
In a case where a power supply switch of the TV monitor is turned on and where signals derived from respective vehicle components indicate that the vehicle stops safely, the liquid crystal filter is in an off state (no voltage is applied across the surface electrode). It is noted that the safe stop state means that: 1) vehicle speed is zero (vehicle speed indicating signal indicates that the vehicle speed is zero); 2) a hand brake (also called parking brake) placed on a side of the driver's seat is operated (sufficiently pulled up toward the driver); and, 3) a gear position indicating signal derived from an automatic power transmission indicates that the gear lever thereof is placed on a parking position or neutral position.
As described above, the safely stopped state is defined as a state in which the three conditions are satisfied.
In this way, the liquid crystal filter control/drive circuit receives no voltage when the three conditions are satisfied and the power supply switch of the TV monitor is turned on.
When the liquid crystal filter is in the off state, light is transmitted in an oblique direction in addition to other directions allowing both passenger and driver to view TV images through the liquid crystal filter.
On the other hand, even though the TV monitor is turned on, the liquid crystal filter control/drive circuit causes the liquid crystal filter to be in the on state (a predetermined voltage is applied) if any one of the three conditions is not satisfied. If the liquid crystal filter is in the on state, light in a direction toward the eyes of the driver's position from the liquid crystal filter which is the maximum contrast direction of the liquid crystal filter, becomes polarized (black), the TV images cannot be viewed from a position at which the driver's eyes are situated.
At this time, since the angle of field through which the polarization occurs is narrow, the liquid crystal filter can transmit the light from the TV monitor toward the passenger's seat side and the passenger can view the TV images.
As described above, the driver can view the TV images through the liquid crystal filter only when the vehicle safely stops. Consequently, safe stopping practices by the driver can be assured and improved.
In addition, to prevent the TV image from being viewed through a front windshield, a louver type filter is installed between the liquid crystal filter and screen of the TV monitor.
The louver type filter includes a multiple number of fine louvers (aligned in a window blind form) placed in a transparent plate (for example, a plate made of synthetic resin).
When the louver type filter is installed on the front portion (screen) of the TV monitor so that elongate direction of the louvers of the louver type filter coincides with the horizontal direction and the light is impinged on a rear side of the louver type filter, a light transmission area is limited only in an area between an oblique upper direction (about 45 degrees upward with respect to the horizontal direction) and an oblique lower direction (about 45 degrees downward with respect to the horizontal direction). The directions except the transmission area described above is defined as a light interruption area. Hence, if the louver type filter is disposed so that an upper light interruption area is placed between the liquid crystal display and the front windshield, i.e., the elongate directions of the louvers are disposed so as to align in a substantially horizontal direction, the light derived from the TV image cannot be directed toward the front windshield.
FIG. 1 shows a characteristic graph of a relationship between viewing position and the transmittance of the filters in the previously proposed TV image displaying apparatus described above.
As appreciated from FIG. 1, an overall transmittance of the filters, i.e., the liquid crystal filter and louver type filter is the addition of the transmittance of the liquid crystal filter and that of the louver type filter. However, since the light transmittance is high at the passenger's seat position, a passenger sitting in the front passenger seat can sufficiently view the TV screen images. On the other hand, since the light transmittance is remarkably low at the driver's seat position and the area in which the driver normally sits on the driver's seat wholly falls in the light interruption area, the TV screen images cannot be viewed by the driver during vehicle run.
However, the previously proposed TV image displaying apparatus has a drawback as described below.
That is to say, although, as the light interruption area, i.e., as the area in which the driver cannot view the TV images, an area in which the driver sits to drive the vehicle in a normal driving posture is set, a driver may tend to try to view the TV image with a posture out of the normal driving posture (out of the light interruption area), that is to say, a driver may tend to lean against the passenger's seat position when the vehicle runs at a cruise speed, at a very low speed, and/or during freeway running. This results in an unsafe driving situation which may endanger driver, passenger and the occupants of other vehicles.